The present invention generally relates to packaging and more particularly to a simple apparatus for wrapping a load having a plurality of components to contain the load in a web of stretched film as a unitary package. This simplified apparatus can be used on low value stretch wrapping machines.
Case packing or boxing is a common way of shipping multiple unit products. The multiple unit products are generally stacked in a corrugated box or are wrapped with kraft paper with the ends of the kraft paper being glued or taped. Another way of shipping such products is by putting a sleeve or covering of heat shrinkable film around the products and shrinking the sleeve to form a unitized package. The use of heat shrinkable film is described in U.S. Pat. Nos. 3,793,798; 3,626,654; 3,590,549 and 3,514,920.
The most common method of wrapping loads currently being used is with rotary stretch wrapping machines. These rotary machines are commonly referred to as spiral or full-web machines, and can operate with the load rotating to pull stretched film web around it. Alternatively, the load can be stationary and stretched film wrapped around the load with a rotating film dispenser.
A typical state-of-the-art full-web apparatus is disclosed in U.S. Pat. No. 3,867,806. This patent discloses the use of relatively untensioned film at the beginning of a wrap.
The use of spiral wrapping machinery is well known in the art and representative machines are typified by U.S. Pat. Nos. 3,003,297; 3,788,199; 3,683,425 and 4,136,501.
The film stretching means on all low volume currently marketed pallet stretch wrapping devices employ either direct or indirect friction to restrict the film as it is being wound onto the load during the wrapping process. The restriction is either applied to the roll of film itself (direct friction) or applied to the film after it is unwound from the film roll (indirect friction). The pallet and load serve as the winding mandrel providing all of the pulling force required to elongate the film.
The earliest type of film stretch wrapper utilized a direct friction device in the form of a brake that is connected to the film roll through the core. The torque from the frictional brake device acted on the center of the film roll and as the roll changed diameter, the voltage to the brake was altered, either by the operator or automatically by a sensing device. A later film roll brake device, illustrated by U.S. Pat. No. 4,077,129, utilizes a frictional brake attached to a shaft with a roller which is pressed against the freely mounted film roll. The film roll brake eliminates the need to change the brake force during the consumption of the film roll.
Various prior art indirect friction film stretching devices have been employed to restrict the film as it is wound onto the pallet during the wrapping process. One of these devices, commonly referred to as an "S" type roller device, utilized an idle roller followed by a braked roller over which the film is threaded prior to wrapping the load. The function of the two rollers is to align the film for maximum contact with the braked roller. Another indirect friction device having fixed bars was marketed by a company known as Radient Engineering Corporation under the trade name POS-A-TENSIONER and has been subsequently marketed by the Kaufman Company under the trade name TNT. This device has a series of fixed, non-rotating bars positioned adjacent to the film roll. The film web is threaded around the bars whose relative angles can be changed for ultimate tensioning. As the film web is attached to the pallet, it is drawn across the bars and the friction between the film and the smooth surface of the bars provides a restriction causing the film to stretch. This device uses multiple bars with the film web stretching incrementally between each bar. Neck-down of the film web increases between each bar and the load bears the force. As the load rotates, the wrap angle changes from the last bar so that the wrapping force greatly varies depending on the relative angles. The frictional restraint is determined by the vector of the film web on each bar. Thus, the device is very sensitive to the force placed on the unwind roll and the force increases as the roll size decreases adding additional force on the system. Furthermore, there must be some friction placed on the supply roll to prevent backlash. While this device solves, to some degree, the irregularities of the brake and the hostility of the film roll, it can only apply limited stretch to the load and does not handle different film compositions with any degree of standardization.
Another stretch wrapper device was introduced by the Anderson Company at the PMMI Show in Chicago in 1978. This device interconnected the turntable drive motor with a pair of nip rollers immediately downstream from the film unwind roll. The nip rollers were synchronously driven with the turntable rotation through a variable transmission which could be increased or decreased in speed relative to the turntable rotation speed. Thus, the stretch on the film was affected between the constant speed nip rollers and the pallet turning. It is not known if this machine was ever commercialized, principally because of its inability to achieve satisfactory stretch over the load corners due to its failure to respond to the speed change that these corners represented. The pallet, as the film accumulating mandrel, provided the total force that was required to stretch the film from the driven nip rollers with all of the stretch occurring after the passage of the film from the nip rollers to the pallet.
The aforementioned stretching devices do not maintain a consistent force in stretching the film web. These brake devices are subject to variation due to their physical construction and their sensitivity to speed change caused by passage of corners of the load and the resultant sudden speed-up and slow-down of the film drawn from the feed roll.
The elasticity of stretched plastic film holds the products of the load under more tension than either shrink wrap or kraft wrap, particularly with products which settle when packaged. The effectiveness of stretched plastic film in holding a load together is a function of the containment or stretch force being placed on the load and the ultimate strength of the total lavered film wrap. These two functions are determined by the modulus of hardness of the film after stretch has taken place and the ultimate strength of the film after application. Containment force is currently achieved by maximizing elongation until just below a critical point where rupture of the film occurs. Virtually all stretch films on the market today, including products of Mobil Chemical Company (Mobil-X, Mobil-C and Mobil-H), Borden Resinite Division PS-26, Consolidated Thermoplastics, Presto, PPD and others, are consistently stretched less than thirty percent in most commercial applications despite manufacturer's laboratory rated capacity in excess of 300 percent in most cases.
The problem of obtaining less stretch on commercial wrapping than that available under laboratory conditions centers on several facts. A square or rectangular pallet which is typically positioned off of its center of rotation is used as the wind up mandrel for the purpose of stretching film. A typical 40".times.48" pallet positioned 3 to 4 inches off of its center of rotation will experience a speed change of up to sixty percent within one quarter revolution of the turntable.
In addition to the off centering problem, most pallet loads are irregular in shape with vertical profiles which produce a significant puncture hazard to highly stretched film being wound around them. Further, some unit loads are very susceptible to crushing forces of the stretched film. Because of pallet load changes and inconsistencies within the film roll, the operator typically continues to reduce the tension settings until there are no failures. Thus, the inconsistencies of films, stretching devices, and pallet loads produce an environment where very few stretch films are actually stretched to their optimum yield.
The major problems with current stretch technology are that stretch is produced by frictional force devices to restrict the film travel between two relatively hostile bodies. On the one hand the film roll is subject to edge wandering and feathering, while on the other hand the rotating pallet with its irregular edges and rapidly changing wind-up speed severly limits the level of elongation achieved. The ultimate holding forces of the film cannot be brought to bear on the load because the film cannot be stretched enough. Even if the film could be stretched enough, the high wrapping forces can disrupt or crush many unit loads. The use of high modulus films, such as oriented films, does not produce the yield benefits of the current invention, since these higher modulus films would have to be significantly stretched in order to achieve the rubberband effect and moldability required for irregular loads.
It therefore can be understood, since the pallet provides the forces for stretching the film, that stretch percentages achieved on the pallet and the stretch force achieved are intertwined in all prior art devices. As previously indicated, high stretch percentages are required to achieve the benefits of high yield, but the high stretch forces incurred at these high stretched percentages cause premature film rupture and potential crushing of the load.
A stretch wrapping device known under the trademark "ROLLER STRETCH" is currently manufactured by Lantech, Inc., which utilizes the film web to drive the apparatus. This device, which is more fully described in U.S. Pat. No. 4,302,920, addresses several of the aforementioned problems. Since the film is pre-stretched between the rollers, which is due to the mechanical advantage between the film driven rollers, it isolates the stretching action from between the film roll and the pallet. This device provides a consistent level of stretch, and more importantly responds to force and speed changes of the pallet without complex feedback controls currently required on other pre-stretch devices.
Balance is achieved when elongation between the rollers (E.sub.1) is equal to elongation on the load (E.sub.2). The relatively higher forces between the closely spaced rollers are overcome by the lower force required to drive the device by the film between the roll and the load. The stress/strain curve experienced between closely spaced rollers is substantially higher than the curve where film is allowed to expend the pulling force. Thus, the film to the load effects this higher force between the rollers aided by the mechanical advantage of the differential pulley relationship of the gear connected rollers. At balance point the elongation on the load (E.sub.2) equals elongation between the rollers (E.sub.1) and the mechanical advantage represents the difference between the forces corrected for friction. Balance is achieved on most films of 120 percent or less elongation between the rollers.
It is therefore apparent that there exists a need for an inexpensive pallet load wrapping apparatus which can utilize the benefits taught by U.S. Pat. No. 4,302,920.